Viral infections continue, particularly since the advent of HIV, to exact a massive toll of human morbidity and mortality. This is despite the use of antiviral vaccines, which nevertheless represent one of the most remarkable achievements in medical care over the past three decades, having eradicated smallpox, and diminished the load imposed by many other pathogenic viruses. HIV is the major concern at present, but other agents too must be considered threatening, particularly in underdeveloped countries: e.g. measles still kills around 2 million people annually, polio still paralyses, and influenza is a constant Date Released: threat. To meet these challenges, a full understanding of the immunological basis of vaccine-induced protection must be obtained. In the first period of funding Dr. Whitton's group has demonstrated a protective role for vaccine-induced CD8+ cytotoxic T lymphocytes: a vaccine has been designed which induces these cells without inducing antiviral antibodies, and which confers complete protection against normally-lethal virus challenge. This competing proposal has three aims: first, to further dissect the factors which determine whether a given virus sequence will induce CD8+ CTL responses. The effects of amino-acid residues flanking the epitopes will be determined, and the ability to link multiple short epitopes in a "string-of-beads" vaccine (which "concentrates" the immunologically- critical sequences, thus increasing the effective capacity of many delivery vectors) will be assessed. MHC class I and II, and B cell, epitopes will be included in such a string-of-beads, and the immune responses measured. Second, the role of CD4+ T cells will be measured, exploiting Dr. Whitton's recent collaborative finding that mice unable to mount normal CD8+ CTL responses instead mount a significant CD4+ CTL response. Are CD4+ CTL instrumental in controlling LCMV infection Le are they a somewhat-effective "backup" system? If so, what mechanism underlies this? Thirdly, a comparative evaluation will be undertaken of several antigen delivery systems, to determine which one provides the most balanced and effective antiviral response. Vaccinia virus will be used (continuing and expanding upon the applicants previous studies); in addition, retroviruses and retrovirus-transformed cells will be evaluated (how well do these agents, already approved for human gene therapy trials, induce immunity?); and purified plasmid DNA will be directly injected intramuscularly (this is known to induce responses, but the biological efficacy has never been determined). Furthermore, the effect of administration of pre-formed antigen will be measured, using synthetic peptides (what are the optimal sizes and sequences for induction of effective responses?) and/or immuno-stimulatory complexes (ISCOMs). Immune responses will be evaluated by several criteria.